1. Field of the Invention
The present invention relates to a method for manufacturing a silicon semiconductor thin film having crystallinity which is formed on a substrate having an insulating surface such as a glass substrate.
2. Description of the Related Art
In recent years, attention has been paid to a technique by which a thin-film transistor is formed using a silicon thin film formed on a glass substrate. The thin-film transistor of this type is mainly used for an active matrix liquid-crystal electro-optical device, and other thin-film integrated circuits. The liquid-crystal electro-optical device is designed such that liquid crystal is sealingly interposed between a pair of glass substrates, and an electric field is applied to the liquid crystal, to thereby change the optical characteristic of the liquid crystal, thus conducting image display.
In particular, the active matrix liquid-crystal display unit using the thin-film transistors is characterized by arranging the thin-film transistors as switches for the respective pixels, and controlling charges held by pixel electrodes. The active matrix liquid display unit is used for the display of a variety of electronic equipments (for example, a portable word processor or a portable computer) because it can display a fine image at a high speed.
An amorphous silicon thin film is generally employed for the thin-film transistor used in the active matrix liquid-crystal display unit.
However, the thin-film transistor using the amorphous silicon thin film suffers from problems stated below.
(1) A higher-quality image display cannot be conducted because the characteristic is low.
(2) A peripheral circuit for driving the thin-film transistor disposed in a pixel cannot be constituted.
The above problem (2) can be classified into two problems one of which is that a CMOS circuit cannot be constituted, since a p-channel type thin-film transistor is not put into practical use for the thin-film transistor using the amorphous silicon thin film, and the other is that the peripheral drive circuit cannot be constituted, since the thin-film transistor using the amorphous silicon thin film cannot conduct high-speed operation and also does not allow a large current to flow therein.
As a method for solving those problems, there is a technique by which a thin-film transistor is formed using a crystalline silicon thin film. As methods for obtaining the crystalline silicon thin film, there are a method for subjecting the amorphous silicon film to a heat treatment, and a method for irradiating a laser light onto the amorphous silicon film.
The method for crystallizing the amorphous silicon film through the heat treatment generally suffers from problems stated below. In order to form the thin-film transistor used in the liquid-crystal electro-optical device, the thin-film transistor is usually required to be formed on a translucent substrate. The translucent substrate may be formed of a quartz substrate or a glass substrate. However, the quartz substrate is expensive and therefore cannot be used in the liquid-crystal electro-optical device which suffers from such a technical problem that the costs must be decreased. Hence, although the glass substrate is generally used, it suffers from such a problem that its heat resistant temperature is low.
It has been proved from the experiment that a temperature of 600xc2x0 C. or higher is required to crystallize the amorphous silicon film by heating, and also it has been proved from the experiment that several tens hours are required as a heating period. Such a high-temperature and long-period heating cannot be conducted on a large-area glass substrate.
Also, there has been known a technique in which the amorphous silicon film is crystallized by the irradiation of a laser light. However, it is difficult to uniformly irradiate a laser light over a large area of the film, or to irradiate a laser light while maintaining a given irradiation power as a real problem.
The present invention has been made to eliminate the above problems, and therefore an object of the present invention is to provide a method for manufacturing a semiconductor thin film, which uses the catalytic action of a metal element and manufactures a crystalline silicon film excellent in characteristics.
In order to solve the above problem, the present invention has been achieved by the provision of a method for manufacturing a semiconductor thin film, comprising the steps of:
introducing metal elements into an amorphous silicon film;
crystallizing said amorphous silicon film to obtain a crystalline silicon film;
forming a protective film on said crystalline silicon film;
forming an amorphous silicon film containing impurities therein on said protective film;
diffusing said metal elements in said amorphous silicon film containing the impurities therein; and
removing said amorphous silicon film containing the impurities therein with said protective film as an etching stopper.
In the above method, the amorphous silicon film to be crystallized may be formed of a film which is formed on a glass substrate or a glass substrate on which an insulating film is formed through a plasma CVD method or a low pressure thermal CVD method.
Also, the metal element may be one kind of element or plural kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. Those metal elements have the catalytic action that promotes the crystallization of silicon, and Nickel (Ni) has the particular catalytic action among those metal elements.
A method for introducing the above metal elements may be a method for forming a layer made of the above-mentioned metal or a layer containing the metal therein on the surface of the amorphous silicon film. Specifically, there are methods for forming a metal-element layer or a layer containing the metal elements therein through the CVD method, the sputtering method, the vapor deposition method or the like, and a method for coating solution containing the metal elements therein on the amorphous silicon film.
However, in case of using the CVD method, the sputtering method, the vapor deposition method or the like, since it is difficult to form a very-thin uniform film, the metal elements non-uniformly exist on the amorphous silicon film, which leads to such a problem that the metal elements are liable to locally exist at the time of crystal growth. On the other hand, the method of using the solution is very preferable, since the concentration of the metal elements can be readily controlled, and the metal elements can be held in uniform contact with the surface of the amorphous silicon film.
In order to crystallize the amorphous silicon film into which the metal elements that promote the crystallization of silicon are introduced, heating may be conducted at a temperature of 450xc2x0 C. or higher. The upper limit of the heating temperature is limited by the heat resistant temperature of the glass substrate used as a substrate. In case of the glass substrate, the heat resistant temperature can be regarded as a strain point of glass. For example, because a Corning 1737 glass substrate is 667xc2x0 C. in strain point, the heating temperature can be set to about 620xc2x0 C., and it is proper from the viewpoints of the heat resistance or the productivity of the glass substrate.
Also, in case of using material that withstands even a temperature of 1,000xc2x0 C. or higher such as a quartz substrate as a substrate, the heating temperature can be increased in accordance with its heat resistant temperature. In addition, the higher the heating temperature is, the more excellent crystallinity can be obtained for the film.
In the above constitution, the step of forming the protective film may be a step of forming a silicon oxide film, silicon nitride film, and a silicon oxynitride film through the plasma CVD method. Alternatively, there can be applied a step of oxidizing the surface of the crystalline silicon film by the irradiation of UV rays, heating or the like in the air.
The protective film is adapted to function as an etching stopper and may be a film by which selectivity can be obtained when etching the silicon film. Also, the thickness of the protective film may be about several tens to 100 xc3x85. The reason why the protective film is thus thinned is that the metal elements need to be movable from the crystalline silicon film through the protective film.
Furthermore, in the above constitution, the amorphous silicon film containing the impurities therein is adapted to function as a film for diffusing the metal elements and may be an amorphous silicon film which is formed through the general CVD method. For example, there can be used an amorphous silicon film which is obtained, for example, through a method identical with a method for forming the amorphous silicon film which is a start film of the crystalline silicon film which has been crystallized by heating.
Also, the impurities contained in the amorphous silicon film are at least one kind of element selected from oxygen, carbon and nitrogen. For example, in the amorphous silicon film, the concentration of impurities is set to 1xc3x971019 to 1xc3x971021 atoms cmxe2x88x923 when the impurities are oxygen, and to 1xc3x971017 to 1xc3x971020 atoms cmxe2x88x923 when the impurities are carbon or nitrogen.
Because the silicon film containing the impurities therein is amorphous in quality (for example, having a large number of dangling bonds, defects and so on), the film per se has an effect of gettering the metal elements such as nickel. However, in the present invention, impurities are allowed to be contained in the silicon film so that the amorphous silicon film has more elements of gettering the metal elements. The elements of this type may be impurities, defects caused by binding of Si and the impurities, oxygen between the silicon binding, and so on.
An increase of the concentration of the impurities higher than the metal elements in the crystalline silicon film is effective in capturing more metal elements in the silicon film. Furthermore, it is effective that the amorphous silicon film is made thicker than the thickness of the crystalline silicon film. This is because the volume ratio of the amorphous silicon film to the crystalline silicon film can be increased as the silicon film is thick. Also, that the impurities such as oxygen are contained with high density in the amorphous silicon film enables more defects to be contained in the film when the silicon film is crystallized by heating, with the result that the above metal element can be gettered in the periphery of those defects.
Moreover, as a film by which the metal elements are diffused, an amorphous SixGe1xe2x88x92x film (0 less than xc3x97 less than 1) containing the above impurities therein can be used. In order to obtain amorphous SixGe1xe2x88x92x film, it may be formed through the plasma CVD method using silane (SiH4) and german (GeH4) as raw gas.
In the above method, the step of diffusing (drawing in) the metal elements in the silicon film containing the impurities therein can be conducted by a heat treatment. The metal elements are diffused in the silicon film containing the impurity elements therein by heating. As a result, because, in fact, the silicon film enables the metal elements in the crystalline silicon film to be drawn out, a crystalline silicon film which is low in the concentration of the metal elements and excellent in crystallinity can be obtained.
For example, if the silicon film containing the impurities therein has substantially the same thickness as that of the crystalline silicon film, the concentration of the metal elements in the crystalline silicon film can be set to xc2xd or less in average by heating.
It should be noted that because the above heat treating step is conducted to diffuse the metal elements outside the crystalline silicon film, the lower limit of the heating temperature is defined by a temperature at which the metal elements can be diffused. Since the effect of reducing the concentration of the metal elements in the crystalline silicon film is more enhanced as the heating temperature is high, it is preferable that heating is conducted at a temperature as high as possible. Hence, from the viewpoint of forming the crystalline silicon film on the substrate, the upper limit of the heating temperature is defined by the strain point or lower of the substrate.
It should be noted that the strain point of the substrate is a criterion for setting the heating temperature, and the heating temperature needs to be set to a temperature at which the deformation or the strain of the substrate is permissible. For example, as represented by RTP, heating can be conducted at a temperature higher than the strain point or higher of the substrate if it is a short-period of high-temperature heat treatment. Furthermore, the temperature and period of time necessary for the heat treatment depend on the pattern of the film to be processed, the rule of a design and so on. Hence, the heat treatment may be conducted at 550 to 1,050xc2x0 C. for about several minutes to 10 hours with the above conditions being satisfied.
There is a case in which when the heating temperature is set as high as possible as described above, the silicon film containing the impurities therein is crystallized due to the catalytic action of the metal elements which are diffused from the crystalline silicon film with the result that the silicon film is formed into a crystalline silicon film. Similarly, in this case, even though silicon containing the impurities therein is crystallized as described above, a large number of defects are formed inside of silicon and act as the selective gettering sink.
Because the crystallizing step is progressed while the metal elements are diffused, crystal growth is progressed from the surface of the crystalline silicon film toward the silicon film containing the impurities therein. Because the metal elements such as nickel have a tendency to concentrate on the tip portion of the crystal growth, even though the crystal growth is progressed, the concentration of the nickel elements in the crystalline silicon film can be reduced, and in addition, a region in which the nickel elements are segregated can be eliminated.
It should be noted that because the impurities are contained in the silicon film which allows the metal elements to be diffused, even though the silicon film is brought into crystallinity, two silicon films can be made different in quality from each other through the protective film.
Further, in the present invention, even though the step of diffusing the metal elements in the amorphous silicon film containing the impurities therein is conducted at a temperature at which the amorphous silicon film containing the impurities therein is not crystallized, its effect can be sufficiently obtained.
This is because the impurities such as oxygen are contained at a high concentration in the amorphous silicon film, heating restrains the amorphous silicon film from being crystallized, and the metal elements can be diffused even at a temperature at which the amorphous silicon film is not crystallized.
Further, the heat treatment is conducted for a long period of time at a temperature at which the amorphous silicon film containing the impurities therein is not crystallized, thereby being capable of more reducing the concentration of the metal in the crystalline silicon film although it is gradual. This action is a remarkable characteristic which cannot be found in the case where the amorphous silicon film is crystallized.
Further, because the amorphous silicon film containing the impurities therein is not allowed to be crystallized so that the amorphous silicon film into which the metal elements have been diffused and the crystallized silicon film are different in crystallinity from each other, there can be obtained such an effect that the step of forming the protective film that functions as an etching stopper can be omitted.
The removal of the silicon film containing the impurities therein on the protective film may be achieved by the application of the wet etching method or the dry etching method. In this situation, since the protective film functions as the etching stopper, the silicon film on the protective film into which the metal elements are diffused can be readily selectively etched.
In other words, in the above method, with the formation of the protective film that functions as the etching stopper on the crystalline silicon film, the silicon film on the protective film in which the metal elements exist with a high density can be surely and selectively removed regardless of whether the silicon film on the protective film is crystallized, or not, in the step of diffusing the metal elements.
Hence, in the present invention, in case of forming the protective film, the heating temperature in the step of diffusing the metal elements may be set to a temperature at which the concentration of the nickel elements in the crystalline silicon film can be reduced to a desired value, regardless of whether the silicon film containing the impurities therein is crystallized, or not.
In particular, in the case where the silicon film containing the impurities therein is not crystallized in the step of diffusing the metal elements, the step of forming the protective film that functions as the etching stopper on the crystalline. silicon film can be omitted.
Furthermore, in the above method, when halogen elements are contained in the atmosphere in the heating step of diffusing the metal elements, the gettering effect is more improved.
As a method for introducing the halogen elements, one kind or plural kinds of gases selected from HCl, HF, HBr, Cl2, F2 and Br2 may be used. In general, hydride of halogen may be used.
Since halogen reacts with nickel to form vaporizable halide of metal elements by heating the silicon film in the atmosphere containing halogen elements therein, the action of removing nickel from the crystalline silicon film is more promoted. Similarly, halide of the metal elements is formed on the crystalline silicon film, thereby being capable of making the metal elements in an electrically inactive state. With the introduction of halogen elements, the concentration of the metal elements can be set to {fraction (1/10)} or less at the maximum in comparison with a case in which no halogen element is introduced in the heat treatment.